The accurate detection of minute amounts of chemical and biological substances has been a major goal in bioanalytical technology throughout the twentieth century. Over the years, a wide variety of biosensing strategies have been developed to satisfy numerous needs. The general aim is to develop a scheme that is able to simplify sample preparation steps, provide high selectivity and sensitivity, respond in a continuous and reversible manner, and accomplish measurements without sample perturbation. Added to these conditions are other goals of convenience such as reusability, portability, low costs for mass production, miniaturization, and ease of use. These features are the most demanding parameters for biosensor designs.
The use of fluorescent indicators in bioassays has met with a certain level of success. Many biological molecules show an inherent fluorescence (associated with the amino acid derivatives of tryptophan or tyrosine, nucleic acids, or other metabolites such as porphyrins). Exploiting this inherent property for bioassay is not a simple procedure, however, because the emitted fluorescence signal is generally weak and often difficult to distinguish above a background signal. To overcome these limitations, an additional step of adding fluorescent labels has been used to mark the desired specimen for easier detection. Such addition disturbs the sample, however, and requires extra steps in sample preparation. Other limitations of fluorescence indicators include fluorescence quenching by other solutes and the insensitivity of fluorescence to certain binding events. In current solid-phase immunoassay procedures, wash steps are labor intensive and time consuming when performed manually and would require complicated and, therefore, expensive robotics in an automated format.
Recently, THz-TDS (Terahertz time domain spectroscopy) has emerged as a successful method to probe the electrical properties of thin solid films in the spectral interval from 0.1 to 10 THz, between the infrared and microwave bands. This technique provides a new alternative to measure the refractive index of thin solid films without sample perturbation. The heart of the THz system is a mode-locked Ti:sapphire femtosecond (fs) laser, which generates fs duration pulses at a MHz repetition rate and low average power. A beam splitter separates the laser beam into excitation and reference pulses. The excitation pulse illuminates an unbiased GaAs semiconductor emitter wafer to generate a THz beam, which is collimated and focused onto an electro-optic sampling crystal, <110> oriented ZnTe, with parabolic mirrors. A pellicle after the second parabolic mirror allows the reference beam to travel collinearly with the THz wave across the electro-optic crystal. A quarter wave plate (QWP), a Wollaston prism (P), and a pair of photodiodes are assembled for the balanced detection of the THz beam.
Unfortunately, compared to the existing biosensors in the marketplace, current THz-based systems suffer from poor signal-to-noise ratios and sensitivity. The most useful techniques for monitoring on-off binding in the far-infrared are attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy and Raman spectroscopy. Unfortunately, the complications associated with cryogenic detectors in FTIR and Raleigh lines in Raman are among the anomalies that hamper FTIR and Raman spectroscopy from being widely used. Also, in general, FTIR has a relative poor signal-to-noise ratio for frequencies less than 1 THz.
Therefore, a need remains for a new non-invasive biosensor technology having application, for example, in health care, food monitoring, and weapon detection.